In any modern day circuit design, it is desirable to minimize the use of circuit pins for getting signals on and off an integrated circuit ("IC") chip. A first way of doing this was to change some pins from dedicated input or output pins to pins which could be used for input and output, which are commonly referred to as I/O pins. These were used mostly for the transfer of data on and off an IC chip but were not used particularly as function pins. Function pins were still primarily dedicated pins. Therefore, for pin conservation it is desirable to be able to use an I/O pin also as a function pin. In particular, it is desirable to be able to use an I/O pin for functions such as the selection of a digital electronic format for a selectable logic output stage or other desired functions.
In accomplishing such tasks for an I/O pin, consideration must be taken for problems that will necessarily arise. For instance, there must be a way of preventing the multiple tasks from interfering. Also, the circuits for permitting the I/O pin to perform the multiple function must use minimal IC chip space. The present invention overcomes these problems and provides I/O pins that also are used as function pins which can sense the logic format level selected so as to connect the selected output and disable all others. This selection usually is performed at the IC chip's power-up.